This invention relates to an outboard motor, and more particularly, to an improved outboard motor having a compact configuration with an improved exhaust system having both a main high speed underwater discharge and a low speed exhaust gas discharge opening to the atmosphere.
It is well known in marine engines to silence the exhaust gases emanating from the engine, be it either an inboard or an outboard mounted engine, by passing them through the body of water in which the watercraft is operated. This is normally accomplished by a form of underwater exhaust gas discharge, such as those that discharge the exhaust gases through the hub of the propeller.
At high speed, the boat is typically raised upwardly causing a shallow immersion for the hub of the propeller and which, combination with the rearward motion of the water, facilitates the flow of exhaust gases through the hub of the propeller. However, when the watercraft is operating at low speed, the underwater exhaust gas discharge will become relatively deeply submerged, and this submergence coupled with the relatively low exhaust gas pressure in the exhaust path during low speed operation can give rise to high back pressure in the exhaust system.
High back pressure in the exhaust gas system impedes engine operation at low speeds. For that reason, it is the normal practice to employ a further, low speed, above the water discharge for discharging the exhaust gases directly to the atmosphere without flowing through the body of water in which the watercraft is operating under this low speed running condition.
Normally, the above water exhaust gas discharge is generally open under all conditions, and the flow path to it is provided with restrictions so as to ensure against excess noise when operating under high speed conditions. However, the above water exhaust gas discharge is typically just large enough to handle idling speed exhaust gases yet small enough to cause the bulk of the exhaust gases to become routed to the underwater exhaust at high engine speeds.
Insofar as other aspects of outboard motor design are concerned, conventional outboard motors generally are affixed so that they extend just rearwardly of the transom of a watercraft and include a driveshaft housing in which a vertically positioned driveshaft is journaled for driving a propulsion unit in the lower unit to propel the watercraft. An internal combustion engine is normally mounted in a power head above the driveshaft housing and has its output shaft directly coupled to the driveshaft for driving the propulsion unit.
Although this type of construction may have some advantages, it naturally lends itself to a limited length exhaust scheme, and little space for other features.
In addition to the aforenoted defects of the conventional type outboard motors, there are certain other defects which have been overcome by outboard motors having a construction of the type shown in co-pending U.S. patent application Ser. No. 661,430, filed Feb. 25, 1991, entitled "Exhaust System for Outboard Motor" filed in the name of Susumu Yamazaki, which application is assigned to the assignee hereof now U.S. Pat. No. 5,154,654. That outboard motor obviates the deficiencies of conventional outboard motors by placing the powering engine substantially forwardly of the transom of the watercraft and extending in a horizontally disposed position so that the engine output shaft rotates about a horizontally disposed axis. This configuration provides for an elongate, tuned, water jacketed exhaust system. This advantageous exhaust system pivots about the generally vertically extending steering axis and tilts about the generally horizontally extending pivot axis. The exhaust of the above-mentioned co-pending application is sufficiently silenced and cooled before being expelled in a high speed underwater discharge. Although such an arrangement has excellent advantages, there is some additional advantage to insuring that the exhaust gases produced during idle or at trolling speed are also sufficiently silenced and cooled before being expelled, and that a system for doing so is not accomplished against the significant back pressure inherent in the main exhaust route during idling or at trolling speed.